If you are wiring a standard residential or light-commercial branch circuit (120V or 240V, under 100A), you must use the 60°C column of the copper gauge amperage chart. The baseline, code-compliant picks are: 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, and 6 AWG for 55A. While the wire insulation you buy (like THHN) is rated for 90°C, the terminals on your breakers and receptacles dictate the legal ampacity limit.
This guide provides the complete National Electrical Code (NEC) Table 310.16 data for copper conductors, explains the derating math that modifies these base numbers, and provides a hard decision path to lock in your exact wire size.
How to Read the NEC Gauge Amperage Chart
The most common mistake DIYers and junior apprentices make is looking at the 90°C column because the wire jacket says "THHN 90°C". The NEC does not allow you to use the 90°C column for final ampacity sizing in standard branch circuits.
Per NEC 110.14(C), for circuits rated 100A or less (or marked for 14 AWG through 1 AWG), you must size the conductor based on the 60°C column, unless the equipment is specifically listed and identified for use with 75°C terminations. Most standard residential receptacles, switches, and small breakers are only rated for 60°C terminations. Therefore, the 60°C column is your legal baseline for ampacity. The 75°C and 90°C columns are reserved for specific equipment, feeders over 100A, and derating calculations.
• 15 Amp Breaker: 14 AWG Copper (60°C Column = 15A)
• 20 Amp Breaker: 12 AWG Copper (60°C Column = 20A)
• 30 Amp Breaker: 10 AWG Copper (60°C Column = 30A)
• 50 Amp Breaker: 6 AWG Copper (75°C Column = 65A, but 60°C is 55A, allowing a 50A breaker)
The Complete Copper Gauge Amperage Chart (NEC Table 310.16)
The table below reflects the allowable ampacities for insulated copper conductors rated up to 2000 volts, based on an ambient temperature of 30°C (86°F). Source standard: NFPA 70 (NEC) Table 310.16.
| Wire Size (AWG/kcmil) | 60°C (140°F) Column | 75°C (167°F) Column | 90°C (194°F) Column |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
| 6 AWG | 55A | 65A | 75A |
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 115A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
| 3/0 AWG | 165A | 200A | 225A |
| 4/0 AWG | 195A | 230A | 260A |
Derating: When the Base Chart Fails You
The chart above assumes two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled in a raceway or cable. When you violate either assumption, the wire cannot dissipate heat as efficiently, and you must derate the ampacity.
The 90°C Column Derating Rule
Here is where the 90°C column finally earns its keep. Even though your termination limits you to the 60°C column, NEC 310.15 allows you to use the 90°C column as the starting base for derating calculations, provided the final derated ampacity does not exceed the 60°C (or 75°C) column limit for the termination.
Worked Example: You are pulling 12 AWG THHN copper through a conduit that contains 5 other current-carrying conductors (6 total).
- Step 1: Find the base 90°C ampacity for 12 AWG. The chart says 30A.
- Step 2: Apply the NEC bundling derating factor for 4-6 conductors, which is 80%.
- Step 3: Multiply: 30A × 0.80 = 24A.
- Step 4: Compare to the termination limit. The 60°C column for 12 AWG is 20A. Because 24A is greater than 20A, the wire is still legally permitted to be used on a 20A breaker.
Failure Scenario: If you had 10 conductors in that conduit (50% derating factor), the math is 30A × 0.50 = 15A. Because 15A is less than the 20A termination limit, you must either downgrade the breaker to 15A or upsize the wire to 10 AWG.
What Counts as a Current-Carrying Conductor?
In a standard 120V or 240V single-phase circuit, the grounded (neutral) conductor is not counted as current-carrying for derating purposes. However, in a 3-phase, 4-wire wye circuit where the major load is nonlinear (like LED drivers, computers, or variable frequency drives), the neutral carries harmonic currents and must be counted. Always consult Copper.org building wire guidelines or NEC 310.15(C)(1) for complex multi-wire branch circuits.
What This Chart Cannot Tell You (Voltage Drop)
The gauge amperage chart is strictly a fire safety tool. It tells you the maximum current a wire can carry before the insulation melts or starts a fire. It tells you absolutely nothing about performance.
Wire has resistance. As the run gets longer, voltage drops. The NEC recommends (in informational notes to 215.2 and 310.15) a maximum voltage drop of 3% for branch circuits and 5% for the total feeder + branch combined.
The Math: 12 AWG copper has a resistance of roughly 1.93 ohms per 1,000 feet. If you run a 120V, 20A load (like a space heater) on 12 AWG wire that is 100 feet away from the panel (200 feet total out-and-back), the voltage drop is:
20A × (1.93Ω × 0.2) = 7.72 Volts dropped.
7.72V is a 6.4% drop on a 120V circuit. Your heater will run hot, the motor will struggle, and the wire will run warmer than expected, even though the chart says 12 AWG is "safe" for 20A.
Decision Path: Pick Your Exact Wire Gauge
Stop guessing. Follow this if-then decision tree to arrive at a single, concrete wire size and breaker pairing for your next pull. Assume copper THHN/THWN-2 in conduit or NM-B (Romex) in standard residential walls.
| Condition / Question | Action to Take | Resulting Pick |
|---|---|---|
| 1. Is the load continuous? (On for 3 hours or more, like EV chargers, grow lights, or hardwired heaters) | Multiply the continuous load amps by 1.25 to find the minimum circuit ampacity. | A 32A continuous EV load requires 32 × 1.25 = 40A minimum wire capacity. |
| 2. Is the one-way run longer than 50 feet? (On a 120V circuit) | Upsize the wire 1 AWG larger than the chart dictates for every 50ft past the first 50ft. | A 20A load at 90 feet requires 10 AWG instead of the baseline 12 AWG. |
| 3. Are there more than 3 current-carrying conductors in the conduit? | Apply the NEC bundling derating multiplier to the 90°C column. If the result is lower than your target breaker, upsize the wire. | Four 12 AWG THHN wires in a conduit derate to 24A, which still safely clears a 20A breaker. |
| 4. Does your final calculated ampacity fall between standard breaker sizes? | Per NEC 240.4(B), you may round UP to the next standard breaker size (15, 20, 30, 40, 50, 60), provided the load is not a continuous motor/plug-in cord. | 8 AWG copper (60°C col = 40A) is perfect for a 40A breaker. If using 4 AWG (70A), you can use a 70A breaker if available, or round up to 80A if the load allows. |
| Final Default Pick for Standard 120V/240V Branch Circuits (Under 50ft, <3 conductors): | Use the 60°C column. Match wire to standard breaker. | 15A = 14 AWG | 20A = 12 AWG | 30A = 10 AWG | 40A = 8 AWG | 50A = 6 AWG |
By anchoring your sizing to the 60°C column, applying 90°C derating only when bundling or high ambient temperatures demand it, and upsizing for distance, you ensure your installation passes inspection and performs flawlessly under load.
